The present invention relates to electrical capacitors having synthetic resin film dielectrics.
For many years the electrical industry has sought capacitors that are satisfactory in terms of reliability, dielectric properties, heat resistance, and cost of production. Capacitor dielectric spacers have recently been made of synthetic resin films such as polystyrene films, which are non-metallizable, and polypropylene films, which are easily windable, easily metallizable, and resistant to moisture.
Polypropylene films, either metallized or not, have already been widely used as capacitor dielectric spacers for many applications with or without the use of a dielectric liquid impregnant. However, a particular feature of polypropylene films is the very high amplitude of change, according to the temperature, of the capacity of capacitors made from them. This feature prevents thermally counterbalancing the variations of inductance of materials like ferrites, the temperature coefficient of which is well defined and equal to 120 ppm (parts per million) per degree centigrade. This feature consequently prevents the use of polypropylene films for manufacturing certain types of capacitors, namely those which are present in telephonic channel by-passes.
The temperature coefficient K.theta. of a capacitor is defined as the relative variation of the capacity as a function of the variation of the temperature: ##EQU1##
A material having a temperature coefficient as close to -120 ppm/.degree. C. as possible is therefore required for thermally counter-balancing the inductance variations of ferrites. The temperature coefficient of a material, however, is not uniform throughout the entire temperature range to which a capacitor may be submitted, generally ranging from -55.degree. C. to +100.degree. C. Thus polystyrene has a temperature coefficient of -120 ppm/.degree. C. that remains constant from -55.degree. C. to +75.degree. C., but it cannot be used at temperatures above 75.degree. C. Furthermore, since it is non-metallizable, metallic sheets must be used for forming the capacitor electrodes, which materially increases the volume of the capacitor. As to polypropylene, it exhibits a significant discontinuity of its temperature coefficient according to the temperature range: EQU K.theta..sub.1 =-110 ppm/.degree. C. from -55.degree. to +40.degree. C. EQU K.theta..sub.2 =-340 ppm/.degree. C. from +60.degree. to +100.degree. C.